A boosted engine may offer greater fuel efficiency and lower emissions than a naturally aspirated engine of similar power. During transient conditions, however, the power, fuel efficiency, and emissions-control performance of a boosted engine may suffer. Such transient conditions may include rapidly increasing or decreasing engine load, engine speed, or mass air flow. For example, when the engine load increases rapidly, a turbocharger compressor may require increased torque to deliver an increased air flow. Such torque may not be available, however, if the turbine that drives the compressor is not fully spun up. As a result, an undesirable power lag may occur before the intake air flow builds to the required level.
It has been recognized previously that a turbocharged engine system may be adapted to store compressed air and to use the stored, compressed air to supplement the air charge from the turbocharger compressor. Accordingly, U.S. Pat. No. 5,064,423 describes a system in which compressed air is stored in a boost tank and is dispensed when insufficient compressed air is available from the turbocharger compressor.
However, the inventors herein have recognized that other transient control issues may occur during decreasing engine load. For example, when a throttle valve in a boosted engine system closes, the compressed air charge upstream of the throttle valve is released to the atmosphere to avoid compressor surge. This may be done by opening a compressor by-pass valve, for example. Such actions erode fuel efficiency, however, as the mechanical energy used to compress the air charge is wasted when the air is released to the atmosphere. Moreover, in engine systems equipped with low-pressure (LP) exhaust-gas recirculation (EGR), merely opening the by-pass valve may not adequately prepare the engine for low-load operation. This is because the intake air charge will be diluted with exhaust gas during mid- to high-load operation. When the throttle valve closes, this exhaust gas remains trapped behind the throttle valve. During closed-throttle conditions, however, non-diluted, fresh air may be required for reliable combustion. Thus, even applying the applying the approach of U.S. Pat. No. 5,064,423, combustion instability during closed-throttle conditions may still occur.
The inventors herein have further recognized that a properly configured compressed-air management system can be used to address both of the transient-control issues identified above. Therefore, one embodiment provides a method for providing air to a combustion chamber of an engine, the engine including a compressor and a boost tank selectably coupled to an intake manifold. The method comprises varying a relative amount of engine exhaust in air pressurized in the boost tank based on engine operating conditions, and discharging the air pressurized in the boost tank to the intake manifold. In this manner, air containing an appropriate relative amount of exhaust gas may be stored and later supplied to the engine intake during closed- and open-throttle conditions. For example, by varying the relative amount of exhaust gas in the stored air, it is possible to provide a desired mixture of stored gas, which may be different depending on whether the gas is to be used during open- or closed-throttle conditions.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted herein.